This invention relates generally to electrically small top loaded monopole antennas. It is well known that the electrical efficiency of antennas whose maximum dimensions are a small fraction of the wavelength tends to be poor. Moreover, the instantaneous bandwidth of electrically small antennas tends to be relatively narrow so that continuous tuning is often required to establish resonance of the antenna at the frequency of operation. Thus, wide instantaneous bandwidth, high efficiency, and compactness tend to be conflicting requirements in the antenna art.
The present antenna structure provides a good compromise between these three factors. Small antenna size is accomplished which is desired for convenience and to enhance ruggedness. High efficiency eliminates the need for excessive transmitter power and improves signal-to-noise ratio during reception. Moderately wide instantaneous bandwidth simplifies the design of associated tuning and matching networks.
For electrically small antennas such as this, 0.1 .lambda. (wavelength) or less, one would expect at least 20 or more tuning bands to be needed with concomitant number of matching devices needed, such as in the Army AS-1729 antenna. By virtue of the simple, inexpensive design of this structure however, the matching is greatly simplified to perhaps less than 15 bands required. In addition to the simplicity of the matching networks, improvements in compact size and reduced height of the antennas are achieved, owing to the unique construction shown here for the top-load, vertical elements, and grounding schemes.
The use of capacitive top loading and inductive loading is well known to those skilled in the art. For example, U.S. Pat. No. 3,909,830 issued Sept. 30, 1975, and entitled "Tactical High Frequency Antenna", discloses use of such means. The use of an adjustable top load capacitance is disclosed in U.S. Pat. No. 3,530,470 issued Sept. 22, 1970. The use of adjustable cable chokes is taught in U.S. Pat. No. 2,913,722 issued Nov. 17, 1959. This invention is directed to improvements thereover.
Reference is made to the following related applications: "Small Broadband Antennas Using Lossy Matching Networks" by Charles M. DeSantis, Watson P. Czerwinski, Michael W. Begala, Albert H. Zennella and John C. Wills, Ser. No. 142,917, filed April 23, 1980.